Remove the Harddrive and CDROM and see if it will boot just from a Floppy Disk in a floppy drive . If it will not, disconnect the CMOS battery overnight and see if it will boot from the floppy then. Youwill have to check with someone who has worked on the 5100 and get them to tell you how. Some models have a small white plug on the motherboard with a 2 wire connectoer, but others connect through the Touchpad connector. That should reset your BIOS to default with the floppy drive first
This following was not on a 5100, but rather on 2 Inspiron 8000 M'boards. The BIOS would boot to the setup screens and those showed the CDROM, Floppy and Harddrive as present and recognized, BUT it would also report no Bootable Devices Detected and that is as far as it would go. A good BIOS chip was substituted for each of them in the socket and everything became normal. The bum chips both had A02 version(early) on them. I then installed the good chip in the socket just far enough to boot up the floppy drive and a I8000A23 Flash Diskette to an A: prompt, typed in the Flash command and entered it, and got to the screen that requires a "Y" to continue. I popped out the good chip with a pair of wooden toothpicks(no metal) put in a bad chip, and hit Y. A normal flash to A23 resulted using that procedure with both chips., and all the devices became bootable.
I think you can see the problem is how to get the Floppy to boot up in order to flash the BIOS. Right now you have a Catch 22 situation. You need to use it to flash the BIOS to correct the problem thatis preventing it from booting so you can flash the BIOS!! BROTHER!!